1. Field of the Invention
The present invention relates to a microwave/millimeter wave transmitter device having a frequency up convertor, and more particularly to an ALC (automatic level control) circuit for stabilizing the output level of the transmitter device.
2. Description of the Related Art
The structure of a conventional transmitter device having an ALC function (Japanese Patent Application Laid-open No. Hei 4-157927) is shown in FIG. 5. An inputted intermediate frequency signal (IF signal) is mixed with the output of a local oscillator 20 by a mixer 21 so as to be converted into a radio frequency signal (RF signal), and then amplified by a variable gain amplifier 22 and a power amplifier 23, thus being outputted as an RF signal. The RF signal transmission output level is detected by a detector 24, and then sent to the variable gain amplifier 22 through an automatic gain control circuit (AGC) 25, where the gain of the variable gain amplifier 22 is controlled to make the RF signal transmission output level constant.
However, the above-described transmitter device is expensive because the variable gain amplifier 22 of an RF-band is employed. Also, because automatic gain control is conducted after frequency conversion, in the transmitter device that alters the transmission frequency by use of the local oscillator of the synthesizer system, the variable gain amplifier 22 is required to provide an excellent frequency characteristic in a wide band within the limit of all the gain variations. In addition, when the IF input is interrupted, the variable gain amplifier 22 has the maximum gain, and the leakage of the local signal from the mixer 21 is greatly amplified, resulting in such a problem that the spurious characteristic of the RF output is deteriorated.
In view of the above, a transmitter device shown in FIG. 6 has been known as an improvement in the above-described conventional transmitter device. This transmitter device inputs an IF input signal to a variable gain amplifier 30 to conduct a predetermined amplification, and mixes it by a mixer 32 with the output of a local oscillator 31 to convert it into an RF signal. Then, the transmitter device amplifies the RF signal thus converted is amplified by a power amplifier 33 to extract a transmission output.
In the above transmitter device, the output of the power amplifier 33 is partially branched and detected by a detector 34 to detect an RF transmission output level. The automatic gain control circuit 35 controls the gain of the variable gain amplifier 30 so that the detection output of the detector 34 becomes a predetermined value, to thereby make the RF transmission output level constant.
Accordingly, the above transmitter device per se can be manufactured inexpensively as compared with the above-described conventional transmitter device. Also, because the automatic gain control is conducted before frequency conversion, the variable gain amplifier 30 can control the gain of the IF signal having a predetermined center frequency. As a result, the variable gain amplifier 30 does not particularly require an excellent frequency characteristic in a wide band. Also, there is advantageous in that when the IF signal input is interrupted, the variable gain amplifier 30 becomes the maximum gain so that there is no case in which the leakage of the local signal from the mixer 32 is greatly amplified, thereby being capable of preventing the spurious characteristic of the RF output from being deteriorated. However, in the above-described two conventional transmitter devices, the signal branched from the output of the power amplifier is detected directly by a diode.
Since the input power to detected voltage characteristic of the general diode is that the forward voltage of the diode increases at a low temperature as shown in FIG. 4, the detected voltage is decreased at a reduced input power. Also, there is a risk that the diode is broken when the input power is inputted at the maximum input or more which is determined by a reverse withstand voltage of the diode junction.
For that reason, a range in which a given detected voltage is stably obtained even if an environmental temperature is varied is very narrow to the degree of about 20 dB (100 times) at most.
With the application of a circuit structure in which the output power can be made constantly a predetermined value by use of an automatic level control circuit (ALC) where a part of the output power of the transmitter device is branched and detected as a power monitor, and then compared with a reference voltage to control a variable attenuator, the reference voltage is varied so that the output power of the transmitter device can be varied. However, a variable range in which the varied output power level can be made constant with respect to the environmental temperature is about 20 dB at most because of the limited characteristic of the above diode, and in the case where the output power is controlled and varied, it is very difficult to obtain the detected voltage of sufficient and stable voltage over a wide high-frequency output power range.